styrofoam/wood/fiberglass sandwich stringer

[Ronnie_P]

Hello all, Im restoring a 21.5ft 1979 Fiberform sportsman. The stringers were made a little differently in this boat and I am replacing them. Originally they were plywood incased in fiberglass on either side of a bottom fuel tank. With a piece of plywood layed over the tops on either side, fiberglassed over, filled with flotation foam, and a molded fiberglass floor insert resting on the transom box and attached to the rub rail. It seems the fiberglass gave most of the strength. I do not wish to use plywood agian. My Idea is to take closed cell foam (The 4X8 blue sheets at Lowes and Home Depot), use liquid nail to sandwich the foam with 1/8 inch pine or fiberglass wall covering (the 4X8 pannels sold in the same locations), sealing the ends with paint or RTV, and then fiberglass over them. Since the foam melts in contact with resin, the wood or roughed up fiberglass and pained ends would act as a barrier and would considerably strengthen the foam. This would be laid out as a box and filled with floatation foam. Anyone ever try this? Do you think its a good idea or just rubbish? I know the fiberglass alone is strong enough because i ran the boat when the stringers were mush and there was no flex whatsoever. Any suggestions would be appreciated. Thanks.

 

[flounderman]

Re: styrofoam/wood/fiberglass sandwich stringer

try the urethane foam, the yellow/brown stuff , it is resistent to fiberglass resin. If you purchase it with the foil backing you need to take a fillet knife and "fillet" the "skin" from the "meat" before you apply resin to it.

 

[Ronnie_P]

Re: styrofoam/wood/fiberglass sandwich stringer

Thanks for the reply, where can the urethane foam be found?

 

[flounderman]

Re: styrofoam/wood/fiberglass sandwich stringer

at most building supply stores- Lowes , Home Depot- in the insulation section. It comes in 4'x8' sheets, it is foil coated. It is very light , handle it carefully and don't step on it.

 

[Coors]

Re: styrofoam/wood/fiberglass sandwich stringer

did you leave the fiberglass encasing?

if so, just fill it with a board, and epoxy.

 

[Bondo]

Re: styrofoam/wood/fiberglass sandwich stringer

Ayuh,.....

It's Your boat Ronnie,... You can rebuild it Any way you'd like,.....
But,.......
Personally,....
I think you're really Over-Engineering it.......

 

[pduquette]

Re: styrofoam/wood/fiberglass sandwich stringer

Hi Ronnie + Guys.
As a previous job I use to man/Build walkin coolers . We use to get urethane foam with Figerglass on both sides . You won't find it at H.D./LOWES ; have to search foam manufacturers industrial suppliers.
I'll call a friemd over there monday for a name if it helps

 

[Ronnie_P]

Re: styrofoam/wood/fiberglass sandwich stringer

Thanks for the info, and yes I'm probably over-engineering it. But then again, I'm an Engineering student... I guess its kinda natural. Also my background is in aeronautics so Im always thinking light, which is not always the best route to take on a boat, I know. I would really like to rebuild the thing with the knowlege that I wont have to do it again, since its kind of a pain in the butt. I had to cut out all of the fiberglass incasing because of the way the plywod was laid in, (there were some splice points that created lumps in the glass). I would like to get the info on fiberglass incased foam you worked with pduquette. If its inexpensive it would save alot of time and labor, also it would need to be 3/4 inch thick. I left all of the groves for the wood so that I could follow the outlines and insert the material back in the groves.. Also, what i had planned on doing with the foam wood sandwich is to create one 24 foot long piece by laying the foam out end to end and glueing the wood overlapping the ends of the foam at different point so that no ends coincide. Then cutting my stringer from this long piece. Its overkill, but if it works it may be the answer to old boats and rot.?

 

[oops!]

Re: styrofoam/wood/fiberglass sandwich stringer

ronnie.....

aeronautical engineer eh??

this could be good...the crossover technoligies as you know are very compatible

there is another product......white...thick..plastic....dont know the name...
youve seen it....chefs use it as a cutting board.....light ...strong...

it was suggested to me by a boat manufacturer...i dont know the properties with the resin...(how it reacts)...but its strong and will never rot. ......in canada the product is avalible at a place called indrustria plactics and paint...
it comes in 4x8 sheets.

worth investigating.

cheers
oops

 

[onenomad]

Re: styrofoam/wood/fiberglass sandwich stringer

Starboard? $$$$$!

 

[fishinaddict]

Re: styrofoam/wood/fiberglass sandwich stringer

Starboard is about 400$ for a 4x8 sheet. I used 1/2 teflon in my boat as a supporting deck, and it was about 150$ for a 4x10 sheet.

 

[BOYS & TOYS]

Re: styrofoam/wood/fiberglass sandwich stringer

i use some coosa on my old crestliner
easy to work with but not cheap http://www.coosacomposites.com/bluewater.html
Mark

 

[flynfirefightertoolman]

Re: styrofoam/wood/fiberglass sandwich stringer

I'm also into aviation... and I know that several aircraft home-built kits use glass over foam principle ... one I'm familiar with is called the "Ruttan Quickie".. do a search and you will find tons of info on-line..
One other idea.. ever thought of using a product like "Trex" deck boards..!! They are intended for a life out doors 60 year life expectancy I believe..! Even inserted into the hull of a boat on its edge.. It will bend and follow the hull contour..! I know some quality lumber suppliers (Canadian Home Centre Lumber Stores) can order any length you require..!!
hope this helps...

 

[erikgreen]

Re: styrofoam/wood/fiberglass sandwich stringer

If you're going to use foam cores for your stringers, you need to use a foam with high compressive and shear strength - pourable urethane foam will work in a 4 lb or higher cubic foot density.

If you're wondering why you need a foam like this for a stringer where the glass is most of the strength, look up "stressed skin construction" and "composite panels" in your aeronautical engineering books. Same concept, you need the core of the "panel" to resist letting the skins move (delaminate) or crush. That's why you also need a strong bond between skin and core... liquid nails won't work too well for that.

If what you want is a solid repair with high strength, do the following:

1) Remove all the old stringer material including fiberglass tabbing
2) Cut new stringer cores to size from plywood (or corecell if you want no wood)
3) Bed the new cores in epoxy putty
4) Glass over the cores with 3 layers of at least 12 oz cloth and epoxy resin. If you use epoxy, no mat is needed. Biaxial glass adds extra strength.

This will give you stringers several times stronger than the original with a minimum of work.

If you want an even more ridiculous amount of strength, you can use carbon fiber instead of glass, and use it over a quality foam core like corecell.

In either case, for strength you should remove the old stringer tabbing even if it's in good shape if you're using epoxy. Making a new epoxy bond to the hull is stronger than using the old poly-poly bond of the existing tabs.

If you're not building for strength, then why make it so complicated?

If you do want strength, use better materials, and if you want cheap and simple use wood and cover it with poly. Building a complex panel like you say will be strong if you use quality materials and build it well... but none of that will matter if you use the existing tabbing or poly resin to put it together... they're the weakest link (other than too-light foam) in all this.

Erik